As is known in the art, a film of an alloy of Sn (tin) and a metal which is nobler than Sn (e.g., an Sn—Ag alloy which is an alloy of Sn and silver), formed by electroplating on a substrate surface, is used for lead-free solder bumps. Sn—Ag alloy plating is typically carried out by applying a voltage between an anode and a substrate surface, which are disposed opposite to each other and immersed in an Sn—Ag alloy plating solution containing Sn ions and Ag ions, thereby forming a metal film of Sn—Ag alloy on the substrate surface. Other than the Sn—Ag alloy, an Sn—Cu alloy which is an alloy of Sn and Cu (copper), an Sn—Bi alloy which is an alloy of Sn and Bi (bismuth), or the like can be used as an alloy of Sn and a metal which is nobler than Sn.
Japanese Patent No. 4441725 discloses an Sn alloy plating method using a soluble anode made of Sn (i.e., Sn anode). The Sn anode is disposed in an anode chamber, which is separated from a cathode chamber by an anion exchange membrane. An Sn plating solution and an acid or a salt thereof are held in the anode chamber, while an Sn alloy plating solution is held in the cathode chamber. Sn ions in the anode chamber are supplied through a (liquid) replenishment line to the Sn alloy plating solution in a plating bath.
Japanese Patent No. 3368860 discloses an Sn alloy plating method in which plating of a workpiece, disposed in a plating bath, is performed using an Sn anode that is isolated in the plating bath by means of an anode bag or box formed of a cation exchange membrane.
Japanese laid-open patent publication No. 2003-105581 discloses an Sn alloy plating method using an insoluble anode made of e.g., titanium. In this plating method, Sn is dissolved from an Sn anode in a dissolution bath that is different from a plating bath (or an electrolytic bath), and the dissolved Sn is supplied to an Sn alloy plating solution.
Japanese laid-open patent publication No. H11-21692 discloses an Sn—Ag alloy plating method which involves the steps of providing an auxiliary bath, having a cathode chamber and an anode chamber separated by a diaphragm or partition so that a substance that would cause deterioration will not diffuse into the cathode chamber, and supplying Sn ions to a plating solution (or an anolyte) held in the anode chamber in the auxiliary bath.
The Sn—Ag alloy plating is generally performed with use of an Sn—Ag alloy plating solution containing a salt of an acid capable of forming a water-soluble salt with Sn ion (Sn2+), e.g., tin methanesulfonate, and a salt of an acid capable of forming a water-soluble salt with Ag ion (Ag+), e.g., silver methanesulfonate.
When Sn alloy plating is carried out with use of a soluble anode (Sn anode), Sn ions dissolve from the Sn anode into an Sn alloy plating solution, and therefore an Sn ion concentration of the Sn alloy plating solution increases. Because of this, it is generally difficult to keep the Sn alloy plating solution at a predetermined Sn ion concentration.
When Sn—Ag alloy plating is carried out with use of an insoluble anode made of e.g., titanium, metal ions (Sn ions and Ag ions) and a free acid, e.g., methanesulfonic acid, are separated from each other with the progress of Sn—Ag alloy plating. The metal ions are consumed by plating, and therefore an acid concentration of the Sn—Ag alloy plating solution gradually increases. In order to compensate for the consumption of the metal ions, it is desirable to replenish the Sn—Ag alloy plating solution with metal ions (preferably metal ions dissolved from a metal) and to adjust the acid concentration of the Sn—Ag alloy plating solution within a preferable range so that good appearance of the resulting metal film and a uniformity of a film thickness can be maintained.
Sn ions are consumed in Sn alloy plating. In the plating method disclosed in the above-mentioned patent publication 3, Sn ions that have been dissolved from the Sn anode are supplied to the Sn alloy plating solution. However, no consideration is given to the concentration of an acid, such as methanesulfonic acid, contained in the Sn alloy plating solution. It therefore appears that, while the Sn alloy plating solution can be kept at a constant Sn concentration, the acid concentration of the Sn alloy plating solution may fall outside a preferable range, resulting in poor appearance of the resulting metal film and non-uniformity of a film thickness.